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Troubleshooting Your Analog Input Signal (App Note)

Not getting the analog input reading you expect?  First see the Test an AIN Channel App Note.  Once you confirm the LabJack is fine and the problem is something else, use this app note to troubleshoot your signal.

DMM Comparison

If your signal changes slowly (e.g. very little change over 1 second), you can often use a DMM (digital multimeter or voltmeter) for comparison.  Connect the DMM at the same time to compare simultaneous readings from the DMM and the LabJack.

  • The DMM leads must be securely connected to the same signal wires or same LabJack terminals.

  • The DMM should be set to DC volts.

  • Test your DMM by measuring VS. It should always be 4.75 to 5.25 volts.

  • If troubleshooting single-ended readings, the DMM should be connected to AINx and GND.

  • If troubleshooting differential readings, the DMM should be connected to the 2 applicable AIN, and note that signals cannot be floating, which is a very common mistake with differential connections.

  • Note whether the readings you see from the LabJack change depending on whether the DMM is connected or not.

  • Note whether the readings from the DMM change depending on whether the LabJack is connected or not.

  • If investigating small errors, realize that some LabJacks are much more accurate than some DMMs.

  • If your signal changes substantially in less than 1 second, you need to use something faster like an oscilloscope (set to DC coupled) for the comparison.

  • If after all these bullet tips the DMM and LabJack still disagree, one common reason is source impedance problems.

Suspicious Values

  • 2.44 or 2.5:  If all your readings are stuck around 2.44 (U3) or 2.5 (T4), your signal is likely exceeding the max input voltage of the low voltage analog inputs on the U3/T4.  This can also be a sign of a floating input.

  • +10, -10, +1, -1, +0.1, -0.1, +0.01, -0.01:  If all your readings are stuck near one of these values, your signal is likely exceeding the max or min input voltage (U6, T7, high voltage inputs on U3-HV, high voltage inputs on T4).  This can also be a sign of a floating input.

  • 1.4:  This happens to be the value returned by some floating analog inputs.  Single-ended inputs on the U12 and high voltage inputs on the U3-HV and T4.

  • 0.000000:  It is not normal to get this exact value every time.  We have seen that this is often returned by custom software that is getting some error but not catching the error.  Or if cal constants are all set to 0.0 for some reason.

  • -9999:  Indicates an error or warning.  Most commonly seen with stream auto-recovery.

  • Integer values in thousands:  Likely you are seeing binary data for some reason.

Too Much Noise

The first step to troubleshoot noise is to determine if you are seeing the expected noise level or more than that.

  • Look at raw voltages rather than scaled values.

  • Use the Ground Test from the Test an AIN Channel App Note to confirm the expected noise level in volts.

  • Connect your signal and note the noise level and compare to the expected noise level.

Once you confirm that your LabJack has the expected noise level, and your signal has more noise than that, troubleshoot what is wrong with your signal if anything.

  • Look at raw voltages rather than scaled values.

  • Remove all connections except power/comm and the one signal you are troubleshooting.

  • Use known software provided by LabJack.

  • To avoid actual changes in the signal, evaluate noise over a short period of time.  For example, if troubleshooting noise with a temperature signal and you look at once per second readings over 60 seconds, you might see variance due to actual changes in temperature not noise.

Output Affects Input

Say you are seeing that controlling some output causes changes in your analog input reading.  Here are various questions/tests that will help us help you narrow down the problem:

  • Which LabJack and how is your input connected and output connected? Remove all extra connections besides power/comm, 1 input and 1 output.

  • What sort of change do you see on the input?  From what voltage to what voltage?

  • Do you see the same thing with a known signal?  Remove your signal and instead jumper the analog input to a known signal.  GND is the best known signal, but if ground is not included in the range of your input (e.g. low voltage inputs on U3/T4), use DACx.

  • Do you see the same thing with known software?  Close all other software and use the Test panel in LJControlPanel to control your output and read your input.

  • Do you see the same thing without load on your output?  For example, say you are using an LJTick-RelayDriver to control a relay that controls a lamp.  Try with nothing, just LJTRD, LJTRD + relay, and finally LJTRD + relay + lamp.

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